In
all microphones, sound waves (sound pressure) are translated
into mechanical
vibrations in a thin, flexible diaphragm. These sound vibrations
are then converted by various methods into an electrical signal which varies in
voltage amplitude and frequency in an analog of the original
sound. For this reason, a microphone is an acoustic wave to voltage modulation
transducer.

Kinds
of microphones

An
Oktava condenser microphone.

In
a capacitor microphone, also known as a condenser microphone, the
diaphragm acts as one plate of a capacitor, and the distance
changing vibrations produce changes in a voltage maintained across the
capacitor plates. Capacitor microphones can be expensive and require a power supply,
but give a high-quality sound signal and are used in laboratory and studio recording
applications.

A
foil electret microphone is a relatively new type of condenser microphone
invented at Bell laboratories in
1962, and often simply
called an electret microphone. An electret is a dielectric
material that has been permanently electrically charged or
polarised. Electret microphones have existed since the 1920s but were considered
impractical, but have now become the most common type of all, used in many applications
from high-quality public address to built-in
microphones in small sound recording devices.
Unlike other condenser microphones they require no polarising voltage, but normally
contain an integrated preamplifier which does require
power (often incorrectly called polarising power). They are frequently phantom powered in sound
reinforcement applications.

A
Dynamic Microphone This
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In
ribbon microphones a thin, corrugated metal ribbon is suspended in a magnetic
field: vibration of the ribbon in the magnetic field generates a changing voltage. Ribbon microphones detect
sound in a bidirectional pattern: this characteristic is useful in such applications
as radio and television interviews, where it cuts out
much extraneous sound.

A
Carbon Microphone This
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A
carbon microphone, formerly used in telephone handsets, is a capsule containing
carbon granules
pressed between two metal plates. A voltage is applied across the metal plates,
causing a current to flow through the carbon. One of the plates, the diaphragm,
vibrates in sympathy with incident sound waves, applying a varying pressure to
the carbon. The changing pressure deforms the granules, causing the contact area
between each pair of adjacent granules to change, and this causes the electrical
resistance of the mass of granules to change (lose contact). Since the voltage
across a conductor is proportional to its resistance, the voltage across the capsule
varies according to the sound pressure.

A
piezo microphone uses the phenomenon of piezoelectricity - the
tendency of some materials to produce a voltage when subjected to pressure - to
convert vibrations into an electrical signal. This type of microphone is often
used to mic acoustic instruments for live performance, or to record sounds in
unusual environments (underwater, for instance.)

A
lav microphone is small microphone, often clipped to an actor's or speaker's
tie or collar and allow for mobile, hands-free use during a performance. They
are most often used in theatrical, public speaking, film and television productions.
Short for lavalier.

Directionality

Omnidirectional

Cardioid

Hypercardioid

Bi-directional

Shotgun

Depending
on various aspects of a microphone's construction, it may be nearly equally sensitive
to sound coming in all directions (an omnidirectional
microphone), or it may be more sensitive to sound coming from a particular direction
(a unidirectional microphone). Between the omidirectional microphone and
the microphone with a cardioid characteristic there should be a "wide" cardioid"
(not printed here). The most common of the unidirectional type is called a cardioid microphone,
because the sensitivity pattern somewhat resembles the shape of a heart; most
vocal mikes are cardioid or hyper-cardioid
(similar to cardioid but with a tighter area of front sensitivity and a tiny lobe
of rear sensitivity.) Some microphones have more complex sensitivity patterns.
Most ribbon microphones are bi-directional,
receiving sound from both in front and back of the element. This type of response
is also known as a figure-8 pattern, because of its shape.

Shotgun
microphones, the most directional form of studio microphone, reserve most
of their sensitivity for sounds directly in front of, and to a lesser extent,
the rear of the microphone. Shotgun microphones also have small lobes of sensitivity
to the left and right. This effect is a result of the microphone design, which
generally involves placing the element inside of a tube with slots cut along the
side; wave-cancellation eliminates most of the off-axis noise. Shotgun microphones
are used most commonly on TV and film sets.

A
parabolic microphone
uses a parabolic reflector
to collect and focus sound waves onto a microphone receiver, in much the same
way that a parabolic antenna (e.g.
satellite dish) does with
radio waves. Typical uses of this microphone, which has unusually focused front
sensitivity and can pick up sounds from many meters away, include nature recording,
outdoor sporting events, eavesdropping, law
enforcement, and even espionage. Parabolic microphones
are not typically used for standard recording applications, because they tend
to have poor low-frequency response as a side effect of their design.

A
microphone with an omnidirectional characteristic is a pressure transducer: the
output voltage is proportional to the air pressure at a given time. On the other
hand, a figure-8 pattern is a pressure gradient transducer; the output
voltage is proportional to the difference in pressure on the front and on the
back side. The result of this is that a sound wave coming from the back will lead
to a signal with a sign opposite to that of an identical sound wave from the front.
Moreover, shorter wavelengths (higher frequencies) are picked up more effectively
than lower frequencies. A microphone with a cardioid directional characteristic
is effectively a superposition of an omnidirectional and a figure-8 microphone;
for sound waves coming from the back, the negative signal from the figure-8 cancels
the positive signal from the omnidirectional element, whereas for sound waves
coming from the front, the two add to each other. A hypercardioid microphone is
similar, but with a slightly larger figure-8 contribution.

Since
directional microphones are (partially) pressure gradient transducers,
their sensitivity is dependent from the distance to the sound source. This effect
is known as proximity effect, a bass-boost at distances of a few centimeters.

Response

A
comparison of the far field on-axis frequency response of the Oktava 319 and the
Shure SM58

Because
of differences in their construction, all microphones will have their own characteristic
responses to sound. This difference in response produces a non-uniform phase and frequency response.
Non-omnidirectional microphones usually have a frequency response which also varies
with the angle of the sound source because the directionality mechanism's effectiveness
is frequency-dependent.

Although
for scientific applications microphones with a more uniform response are desirable,
this is often not the case for music recording, as the non-uniform response of
a microphone can produce a desirable coloration of the sound.

Microphone
techniques

There
exist a number of well-developed microphone techniques used for miking musical,
film, or voice sources. Choice of technique depends on a number of factors, including:

The collection of extraneous
noise. This can be a concern, especially in amplified performances, where audio feedback can be
a significant problem. Alternatively, it can be a desired outcome, in situations
where ambient noise is useful (hall reverberation, audience reaction.)

Basic
techniques

There
are several classes of microphone placement for recording and amplification.

In close miking,
a directional microphone is placed relatively close to an instrument or sound-source.
This serves to eliminate extraneous noise-- including room reverberation-- and
is commonly used when attempting to record a number of separate instruments while
keeping the signals separate, or when in order to avoid feedback in an amplified
performance.

In
ambient or distant miking, a sensitive microphone or microphone
is placed at some distance from the sound source. The goal of this technique is
to get a broader, natural mix of the sound source or sources, along with reverberation
from the room or hall.

Stereo
recording techniques

There
are two essential components that the stereo loudspeakers need to place objects
(phantom sources) in the stereo sound-field between the loudspeakers. These are
level difference
L, the relative loudness, and time-delay difference  t, the difference
in arrival time. The "interaural" signals (binaural ILD and ITD) at the ears are
not the stereo microphone technique signals which are coming from the loudspeakers,
and are called "interchannel" signals ( L and  t). Do not mix these
sort of signals. Loudspeaker signals are never ear signals and vice versa. Read
the header "Binaural recording".

Conventional
stereo recording

In
most recordings on CDs, the stereo effect is a level difference that is created
during the mixing process. The following techniques can be used to capture the
live soundstage.

The
X-Y technique involves the coincident placement of two directional microphones.
When two directional microphones are placed coincidentally, typically at a 90+
degree angle to each other (typically with each microphone pointing to a side
of the sound-stage), a stereo effect is achieved simply through intensity differences
of the sound entering each microphone. Due to the lack of time-of-arrival stereo
information, the stereo effect in X-Y recordings has less ambience. The main advantage
is that the signal is mono-compatible, i.e., the signal is suitable for
playback on non-stereo devices such as radio.

The
Mid-Side (M-S) technique is a special case of X-Y and uses a directional
cardioid or an omnidirectional pressure microphone (M) and a bidirectional (figure-8)
microphone (S), placed at a 90 degree angle to each other with the directional
microphone facing the sound-stage. The outputs of these microphones are mixed
in such a way as to generate sum and difference signals between the outputs. The
S signal is added to the M for one channel, and is subtracted (by reversing phase
and adding) to generate the other channel. M-S has two advantages: when the stereo
signal is combined into mono, the signal from the S microphone cancels out entirely,
leaving only the mono recording from the directional M microphone; additionally,
M-S recordings can be "remixed" after recording to alter or even remove the stereo
spread. The M-S technique with an omnidirectional M microphone is equivalent to
X-Y with two cardioids at a 180-degree angle.

Near-coincident
recording is a variant of the X-Y technique and incorporates interchannel
time delay by placing the microphones several inches apart. The ORTF
stereo technique of the Office de Radiodiffusion TÚlÚvision Franšaise = Radio
France, calls for a pair of cardioid microphones placed 17 cm apart at an angle
of 110 degrees. In the NOS stereo technique of the Nederlandse Omroep Stichting
= Holland Radio, the angle is 90 degrees and the distance is 30 cm. The choice
between one and the other depends on the recording angle of the microphone system
and not on the distance to and the width of the sound source. This technique leads
to a realistic stereo effect and has a reasonable mono-compatibility. These signals
have nothing to do with interaural signals which come only from artificial head
recordings.

The
A-B technique uses two omnidirectional microphones at an especial distance
to each other (20 centimeters up to some meters). Stereo information consists
in large time-of-arrival distances and some sound level differences. On playback,
with too large A-B the stereo image can be perceived as somewhat unnatural, as
if the left and right channel are independent sound sources, without an even spread
from left to right. A-B recordings are not so good for mono playback because the
time-of-arrival differences can lead to certain frequency components being canceled
out and other being amplified, the so-called comb-filtering effect, but the stereo
sound can be really convincing. If you use wide A-B for big orchestras, you can
fill the center with another microphone. Then you get the famous "Decca tree",
which has brought us many good sounding recordings.

Baffled
Omnidirectional technique uses a pair of near coincident omnidirectional microphones
with an absorbtive baffle between them and is closely related to Binaural technique.
Stereo information consists primarily of time of arrival differences between the
microphones and intensity differences from the baffle. The Jecklin Disk (http://www.josephson.com/tn5.html),
described by Juerg
Jecklin, uses of a 30 cm flat circular baffle arranged vertically with the
faces perpendicular to the sound source. Pressure microphones are placed 16.5
cm apart, directly left and right of the disk's center. The KFM Sphere,
described by Gunther
Theile consists of two pressure microphones mounted on opposite sides of a
20 cm sphere. The microphones are mounted flush with the surface and arranged
with the 0-axis perpendicular to the sound source.

Binaural
stereo recording

Binaural recording
is a highly specific attempt to recreate the conditions of human hearing, reproducing
the full three-dimensional sound-field. Most binaural recordings use model of
a human head, with microphones placed where the ear canal
could be. A sound source is then recorded with all of the stereo and spatial cues
produced by the head and human pinnae with frequency dependent ILD (interaural
level difference) and ITD (interaural time difference, max. 630 Ás = 0.63 ms)
ear signals. A binaural recording is usually only somewhat successful, in addition
to being highly inconvenient. For one thing, it tends to work well only
when played back directly into the ear canal, via headphones (no speakers),
as other methods of playback add additional spatial cues. Furthermore, as all
heads and pinnae are different, a recording from one "pair of ears" will not always
sound correct to another person. Also, headphones have a frequency response that
compensates for the fact that the reflections from the pinnae, head and shoulders
strongly affect the frequency spectrum, with the assumption that a recording is
taken with a flat frequency spectrum. Introducing the spectral distortion already
in the binaural recording results in an unnatural frequency spectrum, even when
played through headphones. Finally, as visual cues are generally much more powerful
than auditory cues when determining the source of a sound, binaural recordings
are not always convincing to listeners.